14: Multiple threads

Objects
provide a way to divide a program up into independent sections. Often, you also
need to turn a program into separate, independently-running subtasks.

Each
of these independent subtasks is called a thread,
and you program as if each thread runs by itself and has the CPU to itself.
Some underlying mechanism is actually dividing up the CPU time for you, but in
general, you don’t have to think about it, which makes programming with
multiple threads a much easier task.

Some
definitions are useful at this point. A process
is a self-contained running program with its own address space. A multitasking
operating system is capable of running more than one process (program) at a
time, while making it look like each one is chugging along by periodically
providing CPU cycles to each process. A thread is a single sequential flow of
control within a process. A single process can thus have multiple concurrently
executing threads.

There
are many possible uses for multithreading, but in general, you’ll have
some part of your program tied to a particular event or resource, and you
don’t want to hang up the rest of your program because of that. So you
create a thread associated with that event or resource and let it run
independently of the main program. A good example is a “quit”
button – you don’t want to be forced to poll the quit button in
every piece of code you write in your program and yet you want the quit button
to be responsive, as if you
were
checking it regularly. In fact, one of the most immediately compelling reasons
for multithreading is to produce a responsive user interface.